1. Field of the Invention
This invention relates to a computer system, apparatus and method for an electric power system and, more particularly, to a computer system, apparatus and method for calculating electric power parameters such as, for example, power demand.
2. Background Information
Demand charge is one of the most common factors that utility companies employ to determine the total charge to power users (e.g., owners of apartment complexes, industrial power users). Typically, the utility company employs: (1) the peak demand to calculate the power user's demand charge (i.e., $X/kW times the peak demand (kW)), and (2) the total energy consumption to calculate the power user's energy charge (i.e., $0.Y/kWh times the total energy consumed (kWh)). The main electric meter, for example, of the power user may record for the utility company the peak demand, the date and time of the peak demand, and the total energy consumption over the billing period. In some cases, the power user (e.g., the apartment complex owner) wishes to fairly allocate both the demand charge and the energy charge to power sub-users (e.g., the residents of the individual apartments of the apartment complex).
With respect to the allocation of the energy charge, it is known to employ a personal computer (PC) to digitally address a plurality of monitors for power loads over a communications link to gather data generated by the individual monitors for central processing, and to allocate billing for total energy consumed by each of the loads. See, for example, U.S. Pat. No. 5,315,531.
State of the art monitors for alternating current (AC) power systems incorporate microcomputers for calculating various electrical parameters such as RMS currents and voltages, peak currents and voltages, power, energy, power factor and the like. One such monitor is disclosed in U.S. Pat. No. 5,587,917.
Regarding the allocation of the demand charge, it is known to employ PC software to store a history of energy usage for power sub-users in a database.
It is known for a utility company to regularly output a synchronization pulse which defines the beginning of a plurality of periodic time intervals of interest over which the main electric meter determines the peak demand of the power user.
It is also known to employ monitors for AC power systems which receive the synchronization pulse.
The utility company may output a time synchronization pulse on its power grid every day (e.g., at 12:05 AM) to synchronize the electric meters of the power users. By convention, the utility company may predefine a periodic interval (i.e., a demand window) of time (e.g., 15 minutes in length) for the collection and storage of energy usage values (i.e., total accumulated kWh values) associated with the power users. The demand may be defined, for example, to be the average power (kW) over a predefined period of time. The peak demand may be defined, for example, to be the highest demand value, with the time of the peak demand being the end of the time interval in which the peak occurred.
In the example where the time of the peak demand is at the end of the predefined 15-minute interval of time, the PC software stores the energy usage values in the database for each of the power sub-users at the regular 15-minute intervals (e.g., at 12:05 AM, 12:20 AM, 12:35 AM, . . . 11:50 PM) throughout each day. The PC software, which does not receive the daily time synchronization pulse from the power line, employs a real-time clock to define the time and date. Later, after the end of the billing cycle (e.g., a one-month period), the utility company sends the power user an invoice which provides the time of the peak demand (e.g., 4:50 PM on day seventeen of the one-month billing cycle). In turn, the power user's PC software may determine, from the database, the peak demand at the time of the peak demand for each of the power sub-users and employ the power sub-users' peak demands to fairly allocate the demand charge to the power sub-users. The PC software reads the energy usage values at the beginning and at the end of a fixed demand window and computes the demand usage (kW.sub.DU) as defined by Equation 1: ##EQU1## wherein: kWh.sub.2 is the energy usage value reading (kWh) at time t.sub.2
kWh.sub.1 is the energy usage value reading (kWh) at time t.sub.1 PA1 (t.sub.2 -t.sub.1) is the number of hours (e.g., 0.25 hours) of the fixed demand window
In other cases, the utility company may not output a time synchronization pulse and, hence, the monitor cannot receive the time synchronization pulse. In those cases, the utility company may define a different starting time and/or a different periodic interval than the time and/or interval employed by the PC software. For example, the utility company may employ a regular 15-minute interval and a time synchronization pulse on its power grid every day at 12:12 AM such that the time of the peak, as defined by the utility company, might occur at 4:42 PM on one day of the billing period. The PC software, which does not have access to the time synchronization pulse, stores the energy usage values at the exemplary regular 15-minute intervals (i.e., at 12:05 AM, 12:20 AM, 12:35 AM, . . . 11:50 PM) as discussed above. In that case, the PC software approximates the demand usage by employing the closest demand window (e.g., 4:20 PM through 4:35 PM) as opposed to the actual desired demand window (e.g., 4:27 PM through 4:42 PM) employed by the utility company.
Accordingly, there is room for improvement in computer systems, apparatus and methods which calculate demand usage.